HIGH-RESOLUTION CLOUD FEATURE TRACKING ON VENUS BY GALILEO

The Venus cloud deck was monitored in February 1990 for 16 hours at 40 0 nanometers wavelength by the Galileo imaging system, with a spatial resolution of about 15 km and with image time separations as small as 10 minutes. Velocities are deduced by following the motion of small cl oud features. In spite of the high temporal frequencies capable of bei ng detected, no dynamical phenomena are apparent in the velocity data except the already well-known solar tides, possibly altered by the slo w 4-day wave and the Hadley circulation. There is no evidence, to a le vel of approximately 4 m s-1, of eddy or wavelike activity. The domina nt size of sub-global scale albedo features is 200-500 km, and their c ontrast is approximately 5%. At low latitudes there are patches of blo tchy, cell-like structures but at most locations the markings are stre aky. The patterns are similar to those discovered by Mariner 10 and Pi oneer Venus (M. J. S. Belton et al., 1976, J. Atmos. Sci. 33, 1383-139 3; W. B. Rossow et al., 1980, J. Geophys. Res. 85, 8107-8128). Scaling arguments are presented to argue that the mesoscale blotchy cell-like cloud patterns are caused by local dynamics driven in a shallow layer by differential absorption of sunlight. It is also argued that mesosc ale albedo features are either streaky or cell-like simply depending o n whether the horizontal shear of the large scale flow exceeds a certa in critical value. (C) 1994 Academic Press, Inc.